1. Field of the Invention
This invention relates generally to mobile telephone systems and, more particularly, to a method of dynamically controlling the data rate limit on the reverse link of a high data rate transmission system.
2. Description of the Related Art
A communication system is normally required to support a variety of applications. One such communication system is a Code Division Multiple Access (CDMA) system which allows for voice and data communications between end users over a terrestrial link.
The CDMA system is designed to optimize voice communication and many system design parameters are selected to achieve this goal. For example, since time delay between speakers cannot be tolerated, processing delays should be minimized. Each end user is assigned a transmission rate capable of carrying speech data for the duration of a call.
In the CDMA system, end users communicate from mobile stations to base stations located in cells. A user at a mobile station located in a cell communicates with another at a second mobile station or with a standard telephone, by transmitting voice data on the reverse link to a base station. The base station receives the voice data on the reverse link and can route the data to another mobile station via a second base station or to a public switched telephone network. If the second user is at a mobile station, the voice data is transmitted on the forward link of the same base station or of a second base station, to a second mobile station. Otherwise, the voice data is routed through the public switched telephone network to the second user on the standard phone system. In a CDMA system, the forward link and the reverse link are allocated separate frequencies.
The CDMA system is a spread spectrum communication, system where communications from a base station to a cell occur on the reverse link, each transmitting mobile station causes interference to other mobile stations in the network and this interference limits the capacity of the reverse link. The reverse link capacity can be increased by transmitting fewer bits which requires less power and reduces interference when the user is not speaking. To minimize interference and maximize the reverse link capacity, the transmit power of each remote station is controlled to maintain the signal that is received at the cell at a constant level and at a specific Quality of Service (QoS) level as determined by the frame error rate.
The user at each mobile station transmits at a different bit rate depending on the level of speech activity in the conversation of that user. A variable rate speech vocoder provides speech data at full rate when the user is actively speaking and at a low rate during period of silence. The amount of speech activity at any given instant is never constant. Therefor, the total power received at the cell from all transmitting mobile stations varies over time. During periods of active speech, the mobile station transmits at a higher power which causes more interference to other mobile stations which, in turn, increases the probability of frame errors in the voice data received by the cell. This further limits the number of users able to gain access to the communication system to minimize the number of transmitted frames that are lost through excessive interference.
Limiting the reverse link capacity to maintain the desired frame error rate and, therefore, the Quality of Service level has the effect of forcing a cell to operate at less than full capacity. As a result, the reverse link is often under utilized.
Data communication within the CDMA system has different characteristics than voice communication. Data communication is typically characterized by long periods of inactivity, or low activity, interrupted by high bursts of data traffic. An important system requirement for data transmission is the delay required to transfer a burst of data. Transmission delays do not have the same negative impact on data communication as it does for voice communication, but delays are important for measuring the Quality of Service level of the data communication system.
In a CDMA system, since voice communication cannot tolerate extensive delay, priority is given to the transmission of voice data over the transmission of data traffic. The amount of voice activity at any given instant is unpredictable and, therefore, in a CDMA system the transmission of data will be adjusted to prevent the reverse link capacity from being exceeded. In addition, since the mobile station may be in soft handoff between multiple cells, the data transmission rate assigned may be based on the reverse link capacity of each of the base stations involved in the soft handoff.
It is anticipated that the demand for higher transmission rates for data will increase for both the forward link and the reverse link because a typical mobile user is expected to receive and generate increased amounts of data. The forward link signal is the RF signal transmitted from a base station to one or more mobile stations. The reverse link signal is the RF signal transmitted from the mobile station to a base station. In anticipation of the need for a system to transmit data at higher rates, a High Data Rate (HDR) system is evolving.
In a High Data Rate system, the forward link traffic channel is used for the transmission of an end user's data from a base station to a particular mobile station. During normal operation, the base station generates multiple forward link traffic channels, each of which is used for communication with a particular mobile station. The reverse link traffic is used to monitor common channels while exchanging high data rate data, and performs separate registration, authentication and signaling/data exchange with the high data rate system. In the high data rate system, a Rate Limit message is transmitted to control the reverse link Rate Limit for each active mobile station. The Rate Limit sets the constraints for the mobile station medium access control layer protocol, e.g., slow increase of data rate, minimum data rate that can carry the number of bytes in the buffer, and Power Amplifier limits. Thus, the Rate Limit setting is used to tell the mobile what the allowable current maximum transmission rate is for the reverse link. The current algorithm for determining the data rate for the reverse link uses the number of active users in the sector to set the rate limit. When data is transmitted, the forward and reverse link transmission is asymmetric and, therefore, end users that are active on the forward link may not be transmitting data on the reverse link at the same time. Thus, the current method of determining the Rate Limit setting for mobiles on the reverse link does not consider the actual number of mobiles that are currently active only on the reverse link. Therefore, it is not possible with the present method to accurately determine what the rate limit for the mobiles should be to obtain the highest possible data rate that the reverse link can carry. A new method which more accurately determines the rate limit for the reverse link is needed, and this invention is directed towards doing that.